rule for layers

.cursor/rules/creating-rules.mdc

@@ -1,6 +1,6 @@
 ---
 description: How to create and maintain project rules
-globs:
+globs: .cursor/rules/**
 alwaysApply: false
 ---
 
@@ -76,3 +76,4 @@ Concrete guidance with examples.
 | `events` | Typed event bus — emitting and listening to node and grid events |
 | `node-schemas` | Zod schema pattern for node types, createNode, updateNode |
 | `spatial-queries` | Placement validation (canPlaceOnFloor/Wall/Ceiling) for tools |
+| `layers` | Three.js layer constants, ownership, and rendering separation |

.cursor/rules/layers.mdc

@@ -0,0 +1,57 @@
+---
+description: Three.js layer conventions — which layer each object type lives on and why
+globs: packages/viewer/**,apps/editor/**
+alwaysApply: false
+---
+
+# Three.js Layers
+
+Three.js `Layers` control which objects each camera and render pass sees. We use them to separate scene geometry, editor helpers, and zone overlays into distinct rendering buckets without duplicating scene structure.
+
+## Layer Map
+
+| Constant | Value | Package | Purpose |
+|---|---|---|---|
+| `SCENE_LAYER` | `0` | `@pascal-app/viewer` | Default Three.js layer — all regular scene geometry |
+| `EDITOR_LAYER` | `1` | `apps/editor` | Editor-only helpers: grid, tool previews, cursor meshes, snap guides |
+| `ZONE_LAYER` | `2` | `@pascal-app/viewer` | Zone floor fills and wall borders — composited in a separate post-processing pass |
+
+Import the constants from their owning packages:
+
+```ts
+// In viewer code
+import { SCENE_LAYER, ZONE_LAYER } from '@pascal-app/viewer'
+
+// In editor code
+import { EDITOR_LAYER } from '@/lib/constants'
+```
+
+## Why Separate Zones onto Layer 2
+
+Zones use semi-transparent, `depthTest: false` materials that must be composited *on top of* the scene without being fed into SSGI or TRAA. The post-processing pipeline in `post-processing.tsx` renders a dedicated `zonePass` with a `Layers` mask that enables only `ZONE_LAYER` (and disables `SCENE_LAYER`), then blends its output into the final composite manually:
+
+```ts
+const zoneLayers = useMemo(() => {
+  const l = new Layers()
+  l.enable(ZONE_LAYER)
+  l.disable(SCENE_LAYER)
+  return l
+}, [])
+
+zonePass.setLayers(zoneLayers)
+```
+
+This keeps zones out of the SSGI depth/normal buffers (which would produce incorrect AO on transparent surfaces) while still letting them appear correctly over the scene.
+
+## Why Separate Editor Helpers onto Layer 1
+
+The editor camera enables `EDITOR_LAYER` so tools and helpers are visible during editing. The thumbnail generator disables `EDITOR_LAYER` so exports show clean geometry without snap lines or cursor spheres.
+
+## Rules
+
+- **Never hardcode layer numbers.** Always use the named constants.
+- **`SCENE_LAYER` and `ZONE_LAYER` belong in `@pascal-app/viewer`** — they are renderer concerns, not editor concerns.
+- **`EDITOR_LAYER` belongs in `apps/editor`** — the viewer must never import it; editor behaviour is injected via props/children.
+- **Zone meshes must set `layers={ZONE_LAYER}`** so they are picked up by `zonePass` and excluded from `scenePass` depth buffers.
+- **Editor helper meshes must set `layers={EDITOR_LAYER}`** so they are invisible to the thumbnail camera and the viewer's render passes.
+- **Do not add new layers without updating this rule** and the post-processing pipeline accordingly.